Hi Maarten,
First, I'd love to see the problem closely, since both buttons ("Open WebUI" and "Recommended Updates") use the same service to navigate you to Gaia portal.
By the way, this web service actually has a logic of appending the port (according to device type), but first it uses the following configuration on the gateway (see image):
Gateway editor -> Platforms Portal -> Platform Administration Web Portal -> Main URL.
If something is still wrong, logically or in terms of usability, I'd love to hear from you so we can improve our product.